As an automatic focus adjusting device conventionally used for a video apparatus such as a video camera, a device based on a so-called hill-climbing scheme (TTL autofocus) is known, which performs focus adjustment by extracting a high-frequency component from a vide signal obtained from an image sensor such as a CCD and driving a photographing lens so as to maximize the high-frequency component. Such an automatic focus adjustment scheme has merits such as requiring no special optical member for focus adjustment and accurately adjusting the focus at a long or short distance, i.e., regardless of distance.
A case wherein this type of automatic focus adjustment processing is used for a lens-exchangeable video camera will be described with reference to FIG. 8.
Referring to FIG. 8, reference numeral 801 denotes a focusing lens, which is moved in the optical direction by a lens driving motor 811 to perform focus adjustment. Light passing through this lens is formed into an image on the image sensing plane of an image sensor 802 and is photoelectrically converted into an electrical signal. This signal is output as a video signal. This video signal is sampled/held and amplified to a predetermined level by a CDS/AGC 803. The resultant signal is converted into digital video data by an A/D converter 804. The data is input to the process circuit of the camera to be converted into a standard television signal and input to a bandpass filter 805 (to be referred to as a BPF hereafter). The BPF 805 extracts a high-frequency component from the video signal. A gate circuit 806 extracts only a signal corresponding to a portion set in a focus detection area in a frame, and a peak hold circuit 807 holds peaks at intervals synchronized with an integral multiple of a vertical sync signal, thereby generating an AF evaluate value. This AF evaluate value is input to a main body AF microcomputer 808, which in turn determines a motor driving direction so as to increase the focusing speed and AF evaluate value in accordance with the degree of focusing. The speed and direction of the focusing motor are sent to a lens microcomputer 809. The lens microcomputer 809 performs focus adjustment by causing a motor 811 to move the focusing lens 801 in the optical axis direction through a motor driver 810 in accordance with an instruction from the main body. AF microcomputer 808.
According to the above prior art, the camera body incorporates a control mechanism for automatic focus adjustment, and determines an automatic focus adjustment response characteristic or the like so as to optimize the focus within a limited specific lens unit range. When this limited lens unit is exchanged with a lens unit other than the limited lens unit, it is difficult to realize the optimal performance of the camera.
In contrast to this, in a lens-unit-exchangeable camera system, there is provided a technique of generating AF evaluation information instead of generating an AF driving signal for the camera body, and making each lens unit perform focusing independently of the camera body (e.g., Japanese Patent Laid-Open No. 9-9130).
In the prior art described in the above reference, however, the following problems arise.
First of all, if the time frames in which a camera body and a lens unit have been developed are relatively close to each other, no considerable problem arises because the position resolution of the focusing lens in the lens unit is determined in accordance with the specifications of the image sensing system of the camera in which the lens unit is mounted.
However, image sensors mounted on recent cameras have increased in miniaturization and packing density. If, therefore, a succeeding device (let's say, the second device) is to be developed several years after the development of the first device, an image sensor which has increased in miniaturization and packing density as compared with the previous generation is selected as an image sensor used for the second device. In other words, the pixel pitch and permissible circle of confusion of the image sensor of the second device are reduced.
If, therefore, an old lens unit in which the position resolution of the focusing lens is determined in accordance with the permissible circle of confusion of the first device is to be mounted on the camera body of the second device to be developed, since the permissible circle of confusion of the second device is small, a video taken by the device is influenced by movement such as wobbling caused by the micromotion of the focusing lens when autofocus (AF) operation is performed.
In addition, even if the permissible circle of confusion determined by the pixel pitch of the image sensor of the camera body is the same as that of the first device, the effective pixel density of a sensed image increases, and the effective permissible circle of confusion decreases, resulting in the occurrence of a phenomenon similar to that described above, when, for example, a single image sensor (CCD) system is replaced with a triple-CCD system, or when a so-called “pixel shifting” system is used, in which the mount positions of the respective RGB CCDs are shifted from each other by a distance corresponding to ½ pixel in the triple-CCD system, and pixel interpolation is performed to increase the resolution.